Apple·Software Engineer·Onsite - System Design / Architecture·Senior

Senior

Jun 2026

Summary

Apple hardware engineer interview with a product-design exercise focused on thermal modeling. One meaty technical question that took up most of the session, very applied, less theory and more 'show me you can build something real'.

Questions Asked (1)

Design a module that models a thermal conduction system using an electrical resistance analogy. Walk through how you'd define the thermal resistances (conductive, convective, radiative), set boundary conditions, handle heat sources, and describe how the module's interface and parameters would plug into a larger composed thermal system.

System DesignTechnical Trade-offs

Author's notes

This took me a second to orient to because I kept wanting to jump straight into equations.

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AI HintsAI Generated

Suggested Approach

Start by framing the problem as a software design exercise: define a clean abstraction for thermal elements using the electrical resistance analogy, then show how they compose into a system. Walk through the physics-to-code mapping for each resistance type, boundary conditions, and heat sources, emphasizing modularity and interface design for integration into larger systems.

Pro tip: Demonstrate awareness of numerical stability and performance trade-offs: for example, radiative resistance is nonlinear (depends on T^4), so you might linearize it or use iterative solvers, and you should mention how your module handles these without breaking the abstraction.

1. Define the abstraction and core interfaces

Outline a base class or interface for thermal elements (e.g., ThermalElement) with methods to compute resistance and heat flow, and a ThermalSystem class that assembles elements and solves the network. Emphasize that each element exposes a consistent interface for pluggability.

2. Model the three resistance types

For conduction, use R = L/(k*A); for convection, R = 1/(h*A); for radiation, use a linearized resistance R = 1/(h_rad*A) where h_rad = εσ(T_s^2 + T_sur^2)(T_s + T_sur). Explain how each is implemented as a subclass or strategy, and note the nonlinearity of radiation.

3. Handle boundary conditions and heat sources

Represent fixed-temperature boundaries as voltage sources and heat sources as current sources in the analogous circuit. Describe how to incorporate them into the system matrix, and how to handle time-dependent or spatially varying sources via parameters.

4. Design the system assembly and solver

Show how to build a thermal network by connecting elements in series/parallel, forming a system of equations (e.g., using nodal analysis). Discuss solver choices (direct vs. iterative) and how to handle nonlinearities (e.g., Newton-Raphson for radiation).

5. Define the module's public API and integration points

Specify how the module exposes parameters (e.g., material properties, geometry) and results (temperatures, heat fluxes) to a larger composed system. Discuss how it can be nested or coupled with other modules (e.g., via shared boundaries or co-simulation).

Key Points to Mention

Electrical-thermal analogy: temperature ↔ voltage, heat flow ↔ current, thermal resistance ↔ electrical resistance.
Conductive, convective, and radiative resistance formulas and their implementation as interchangeable strategies.
Boundary conditions: fixed temperature (Dirichlet) and fixed heat flux (Neumann), and how they map to voltage/current sources.
Heat sources: internal generation or external flux, modeled as current sources in the network.
System assembly: nodal analysis, matrix construction, and solver selection (direct/iterative, linear/nonlinear).
Modularity and interface design: clear separation of concerns, parameterization, and composition for larger systems.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.